1. Field of the Invention
The invention relates to optical disk drives and more particularly to their tracking servos.
2. Brief Description of the Prior Art
U.S. Pat. No. 4,037,252 issued to U.S. Philips Corporation is incorporated by reference. This patent describes a servo system comprising three laser beams striking an optical disk and from which two radially varying sinusoidal signals are recovered, one offset from the other by 90 degrees. A first signal comprises a difference signal from two offset beams and has a zero crossing at track center. This signal is provided as an error signal to the disk drive's servo system so that it might servo on the zero crossing. The first signal's scaled derivative is subtracted from the first signal to the provide for positive damping. Between tracks, the damping derivative is negative. This ordinarily would cause servo system instability. However, here, the second signal, offset from the first by 90 degrees, is wholly negative and when shaped into a square wave provides a control signal for inverting the damping derivative signal so that damping again is positive.
U.S. Pat. No. 4,456,981 issued to Magnetic Peripherals Inc. is incorporated by reference. This patent discloses offset locations for providing tracking and track crossing information in an optical disk drive. When one crosses the drive's tracks radially, the radially adjacent offset locations combine to produce a signal having flattened peaks in the radial dimension.
U.S. Pat. No. 4,428,069 issued to Burroughs Corporation describes an enlarged timing pit followed by elongated, narrow, staggered offset servo pits spaced close to the center of a track.